How do you do attic vent and roof ventilation calculations for a real house?
Example 1: ranch house with ridge vent and soffits
A 2,000 square foot attic on the balanced path needs 2,000 divided by 300, or about 6.7 square feet of net free area. Times 144, that is about 960 square inches, split into 480 of exhaust and 480 of intake.
- Exhaust: a ridge vent rated 18 square inches per foot needs about 27 feet of open slot. A vent rated 12.5 needs about 39 feet.
- Intake: 480 square inches of soffit vent area, using the rating on the soffit panels or undereave vents. If the soffit falls short, reduce the exhaust to match rather than leaving intake starved.
Example 2: hip roof with box vents
Same 2,000 square foot attic, but the ridges are short. With box vents rated around 50 square inches each, 480 divided by 50 is about 10 vents, placed high on the rear and side slopes, all within 3 feet of the ridge measured vertically so they count as upper vents.
Example 3: older house with only gable vents
Gable louvers sit high, but with no low intake the split is not balanced, so the 1/150 ratio applies. That is 2,000 divided by 150, about 13.3 square feet, or roughly 1,920 square inches. Most gable louvers fall far short of that, which is why these attics run hot. Adding soffit intake and switching to ridge exhaust lets you use the smaller 1/300 figure.
Rules that keep the math honest
- Use attic floor area, not roof area.
- Use net free area from the label, not the vent's outside size.
- Intake should equal or exceed exhaust.
- Count each separate attic space on its own.
Background on the ratios is in roof ventilation basics, and ridge vent vs box vents covers choosing the exhaust type.